c# printing barcode Part II Designing Types in .NET framework

Generator QR in .NET framework Part II Designing Types

Part II Designing Types
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// This shows how to walk through the array of anonymous types (var is required) foreach (var person in people) Console.WriteLine("Person={0}, Year={1}", person.Name, person.Year);
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Anonymous types are most commonly used with the Language Integrated Query (LINQ) technology, where you perform a query that results in a collection of objects that are all of the same anonymous type . Then, you process the objects in the resulting collection . All this takes place in the same method . Here is an example that returns all the files in my document directory that have been modified within the past seven days:
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String myDocuments = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments); var query = from pathname in Directory.GetFiles(myDocuments) let LastWriteTime = File.GetLastWriteTime(pathname) where LastWriteTime > (DateTime.Now - TimeSpan.FromDays(7)) orderby LastWriteTime select new { Path = pathname, LastWriteTime };// Set of anonymous type objects foreach (var file in query) Console.WriteLine("LastWriteTime={0}, Path={1}", file.LastWriteTime, file.Path);
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Instances of anonymous types are not supposed to leak outside of a method . A method cannot be prototyped as accepting a parameter of an anonymous type because there is no way to specify the anonymous type . Similarly, a method cannot indicate that it returns a reference to an anonymous type . While it is possible to treat an instance of an anonymous type as an Object (since all anonymous types are derived from Object), there is no way to cast a variable of type Object back into an anonymous type because you don t know the name of the anonymous type at compile time . If you want to pass a tuple around, then you should consider using the System.Tuple type discussed in the next section .
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The System.Tuple Type
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In the System namespace, Microsoft has defined several generic Tuple types (all derived from Object) that differ by arity (the number of generic parameters) . Here is what the simplest and most complex ones essentially look like:
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// This is the simplest: [Serializable] public class Tuple<T1> { private T1 m_Item1; public Tuple(T1 item1) { m_Item1 = item1; } public T1 Item1 { get { return m_Item1; } } }
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// This is the most complex: [Serializable] public class Tuple<T1, T2, T3, T4, T5, T6, T7, TRest> { private T1 m_Item1; private T2 m_Item2; private T3 m_Item3; private T4 m_Item4; private T5 m_Item5; private T6 m_Item6; private T7 m_Item7; private TRestm_Rest;
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10 Properties
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public Tuple(T1 item1, T2 item2, T3 item3, T4 item4, T5 item5, T6 item6, T7 item7, TRest t) { m_Item1 = item1; m_Item2 = item2; m_Item3 = item3; m_Item4 = item4; m_Item5 = item5; m_Item6 = item6; m_Item7 = item7; m_Rest = rest; } public public public public public public public public } T1 Item1 { T2 Item2 { T3 Item3 { T4 Item4 { T5 Item5 { T6 Item6 { T7 Item7 { TRest Rest get { get { get { get { get { get { get { { get return m_Item1; } } return m_Item2; } } return m_Item3; } } return m_Item4; } } return m_Item5; } } return m_Item6; } } return m_Item7; } } { return m_Rest; } }
Like anonymous types, once a Tuple is created, it is immutable (all properties are read-only) . I don t show it here, but the Tuple classes also offer CompareTo, Equals, GetHashCode, and ToString methods, as well as a Size property . In addition, all the Tuple types implement the IStructuralEquatable, IStructuralComparable, and IComparable interfaces so that you can compare two Tuple objects with each other to see how their fields compare with each other . Refer to the SDK documentation to learn more about these methods and interfaces . Here is an example of a method that uses a Tuple type to return two pieces of information back to a caller:
// Returns minimum in Item1 & maximum in Item2 private static Tuple<Int32, Int32>MinMax(Int32 a, Int32 b) { return new Tuple<Int32, Int32>(Math.Min(a, b), Math.Max(a, b)); } // This shows how to call the method and how to use the returned Tuple private static void TupleTypes() { varminmax = MinMax(6, 2); Console.WriteLine("Min={0}, Max={1}", minmax.Item1, minmax.Item2); // Min=2, Max=6 }
Of course, it is very important that the producer and consumer of the Tuple have a clear understanding of what is being returned in the Item# properties . With anonymous types, the properties are given actual names based on the source code that defines the anonymous type . With Tuple types, the properties are assigned their Item# names by Microsoft and you cannot change this at all . Unfortunately, these names have no real meaning or significance, so it is up to the producer and consumer to assign meanings to them . This also reduces code readability and maintainability so you should add comments to your code explaining what the producer/consumer understanding is . The compiler can only infer generic types when calling a generic method, not when you are calling a constructor . For this reason, the System namespace also includes a non-generic, static Tuple class containing a bunch of static Create methods which can infer generic types from
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